1. Field of the Invention
The present invention relates to a composition containing a carbodiimide resin for an acoustically matching layer, a process for producing a formed article of an acoustically matching layer, and an ultrasonic transducer using the same composition and an ultrasonic transducer using the same formed article, more particularly a composition for an acoustically matching layer excellent in acoustically matching layer characteristics or the like and in stability for use for ultrasonic transducers, a process for producing a formed article of an acoustically matching layer, and an ultrasonic transducer using the same composition and an ultrasonic transducer using the same formed article.
2. Description of the Prior Art
Ultrasonic transducers which use a piezoelectric element to detect the presence or absence of an object or to find the distance to an object have been known. Ultrasonic waves, when traveling through an interface between dissimilar transmitting media, have decreasing transmittance as the difference in acoustic impedance between these media increases. A piezoelectric element, normally made of a ceramic material, has much higher acoustic impedance than air, and is useless for sensing purposes, because ultrasonic waves traveling through air are totally reflected or have greatly decreased transmittance when returning to the element.
For this reason, an ultrasonic transducer is provided with an acoustically matching layer between its piezoelectric element and the air, which has an acoustic impedance intermediate between those of the element and air, to impart a practical transmittance to the ultrasonic waves returning to the element after traveling through air.
An epoxy resin or the like has been normally used for an acoustically matching layer (or composition therefor) for ultrasonic transducers. For example, Japanese Patent Laid-Open Publication No.3-264000 discloses an acoustically matching layer material characterized by woody material for matching impedance between a transducer and medium for ultrasonic transducers, which use a ceramic material or the like. Japanese Patent Laid-open Publication No.8-65795 discloses an ultrasonic transducer, with a piezoelectric element fixed on its acoustically matching layer, characterized by the matching layer composed of an epoxy-based resin or the like mixed with glass balloons (hollow glass particles) and provided with a means for heating the layer. Japanese Patent Laid-open Publication No.10-224895 discloses an ultrasonic transducer which has a λ/4 acoustically matching layer fixed thereon with a piezoelectric element coated on one side with a synthetic resin (e.g., epoxy resin or the like) dispersed with glass balloons.
An acoustically matching layer (or composition therefore) for ultrasonic transducers which emit ultrasonic waves into a gaseous medium, e.g., air, necessarily has a density of around 0.4 to 0.7 g/cm3, and is made of a sintered inorganic material or the like.
However, the acoustically matching layers which have been used or proposed for ultrasonic transducers involve various problems. For example, the one made of a woody material is highly moisture-absorptive and therefore sensitive to environments, and also has a fluctuating specific gravity or density, and it is difficult to control the formation and the like. The one made of a sintered inorganic material is fragile because of the necessity for keeping it at a low density, and hence difficult to handle and process. The one made of a formed resin is unstable in characteristics at high temperature because of the insufficient glass transition temperature and heat resistance of the resin, needs an additional step for preventing contamination with bubbles when glass balloons are kneaded into the resin, and also needs a pretreatment step with a coupling agent or the like because of insufficient adhesion to an inorganic material.